My 96 900M has a whistling gas cap....This occurs with changes in temperature like when the bike is in the sun or parked in the shade....The tank has two vent pipes at the rear and both appear unblocked...I would think any pressure build up in the tank would vent out...I disassembled the cap but nothing looked odd... The engine runs very nicely despite the singing fuel cap...Kinda of annoying, any ideas?
It's doing what it's supposed to. It's venting, and the rubber valves that control airflow are 'talking'.
Try removing the rubber fitting under the cap. See if that helps.